To consider a case where a torsion beam is manufactured in such a manner that a tubular body is manufactured by a normal tube manufacturing method (a tube manufacturing method where the rolling direction of a rolled steel sheet is set to the tub axis direction) and a portion of the tubular body is crushed in the radial direction, there exists a possibility that wrinkles which extend in the tube axis direction (longitudinal direction) are generated on inner peripheral surfaces of edge portions of a crushed portion in the circumferential direction in cross section (hereinafter the edge portions being referred to as “ear portions” since the edge portions have a shape similar to an ear of a rabbit). These wrinkles become a source of fatigue crank initiation and deteriorate the endurance of the torsion beam. To overcome such a drawback, JP-A-2005-289258 proposed the use of a tubular body manufactured by setting the rolling direction of a rolled steel sheet approximately perpendicular to the axial direction of the tubular body as a tubular body provided for crushing and forming in the radial direction and/or the grinding of an inner periphery of the tubular body in the direction approximately perpendicular to the axial direction of the tubular body before or after manufacturing the tube.
JP-A-2002-275538, as a heat treatment method which enhances deformation strength and fatigue strength of car parts, proposes a method where torsion is applied to a steel material within a range where plastic deformation does not take place, heat treatment is applied to a portion to which a tensile stress is applied in such a torsion-applied state, and the torsion is released after cooling the steel material. Due to such a method, it is considered that the direction of the applied compressive residual stress can be easily aligned with the direction of a stress applied to the steel material during use and dimensional accuracy is enhanced due to the suppression of generations of strain in the steel material.
The above-mentioned background art is considered effective as a means which enhances the fatigue strength of the ear portion of the torsion beam which is a high risk part where fatigue crack initiations occur. In forming the portion of the tubular body into a cross section having an approximately U-shape (hereinafter referred to as an approximately U-shape in cross section) by crushing the portion in the radial direction, the ear portion is mainly subjected to bending in the circumferential direction. Hence, a residual stress on a tensile side occurs on a tubular inner surface of the ear portion in the circumferential direction, and this residual stress causes lowering of fatigue characteristics. Although the technique disclosed in JP '258 is the technique which focuses on wrinkles formed on the tubular inner surface which becomes a source of a fatigue crack initiation, the occurrence of wrinkles can be avoided by designing a shape of the ear portion having a slightly larger curvature radius without forming an ear shape having a small curvature radius by which wrinkles occur. A means which reduces a tensile residual stress in the tubular inner surface of the ear portion is important for the enhancement of fatigue characteristics.
However, in the technique disclosed in JP '258, it is necessary to set the rolling direction of the rolled steel sheet substantially perpendicular to the axial direction of the tubular body and/or to grind the inner periphery of the tubular body in the tubular circumferential direction which is the direction approximately perpendicular to the axial direction of the tubular body. In this case, compared to a usual case, the large restriction is imposed on a length of a manufactured tube or the addition of the grinding step is necessary or the like. Hence, there arises a drawback that the technique is disadvantageous in view of productivity and cost. Further, compared to a usual case, the technique disclosed in JP '538 requires the addition of steps such as applying torsion and heat treatment. Hence, the technique also has a drawback that it is disadvantageous in view of productivity and cost in the same manner.